Allow deprecated flags when building with LLVM_NEXT.

-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
is deprecated in the next compiler. So add -Wno-unused-command-line-argument
to skip it.

Bug: 253033919
Bug: 259559364
Test: run test_compiler.py
Change-Id: I9133a36700afd3ef411092fd8133a7fe1c2c7343